[Advanced Dreambooth] Textual Inversion token is weakened/ignored by negative prompt

### Describe the bug

A LoRA + TI token pair trained with the advanced dreambooth script doesn't work with negative prompts.

In my testing, the validations all work perfectly if a negative prompt is not used. When it is included, the subjected trained with the token is often entirely ignored or looks nothing like the original subject.

### Reproduction

1. Train a LoRA + TI with the advanced dreambooth script using an instance prompt like "A photo of TOK".
2. Perform validation prompt like "A photo of TOK" to see valid training results
3. Perform same validation, but with an additional negative prompt like "distorted, disfigured, blurry"
4. Notice the TOK subject is often ignored and when not ignored, retains less characteristics of the original subject

### Logs

Continuing my investigation here: https://github.com/huggingface/diffusers/issues/6685#issuecomment-1908679012

### System Info

Main branch of diffusers

### Who can help?

@linoytsaban